Acid treating well



Sept. 19, 19, N. "B. DISMUKES 2,358,562

ACID TREATING WELL Filed Dec. 6, 1941 Q9 Q PRQDUCTIVITY FACTOR 0,8

B/D/Lb. O

0 IO 20 3O 4O 5O 6O 70 8O 90 I00 VOLUME OF FORMATION DISSOLVED CU. FT.

ATTORNEY Patented Sept. 19, 19

AC TREATING IELL Newton B. Dismukes, Houston, Tex, asslgnor to StandardOil Development Company, a corporation of Delaware Application December6, 1941, Serial No. 421,873

11 Claims.

The present invention is directed to a method of acid treating thedesired formations penetrated by a borehole.

In completing oil wells, it is sometimes necessary and often desirableto treat the producing formations in order to increase or induce theflow of fluid therefrom. For a number of years, acid has been employedfor this purpose, particularly when the producing formations are of alimestone or dolomitic nature, and also occasionally when sandstones areencountered.

It is an object of the present invention to devise a method of acidtreating producing formations in order to induce or increase theproduction therefrom in a more satisfactory manner than has heretoforebeen accomplished.

A further object of the present invention is the stage treatmentofformations with acid in order to obtain funnel-shaped drainage channelscommunicating through the producing formation into the borehole in orderto obtain a substantially constant pressure differential between thereservoir and the bore of the well when producing fluid.

A further object of the present invention is to' utilize a plurality ofacids of differing rates of reaction in sequence to obtain drainagechannels of more satisfactory shape in oil producing formations than hasheretofore been possible and, hence, increasing the productivity of thetreated formations.

Other objects and advantages of the presentinvention may be seen from areading of the following description taken in conjunction with thedrawing in which the sole figure comprises a pair of curves indicatingthe improved results to be obtained in the practice of the presentinvention .as compared with conventional acid treating processes.

The present invention has been found particularly suitable for treatinglimestone, dolomite,

and similar carbonate formations. In conventional acid treatingprocesses employing hydrochloric acid, it has been found that thehydrochloric acid reacts at such a rate that it is sub-.

stantially all consumed in the vicinityof the well bore. This phenomenonis encountered even though one of the known inhibited and, retardedhydrochloric acids is used for the acid treating step. In accordancewith the present invention,

carbonate formations are given a plurality of acid treatments, acids ofdifferent reaction rates being employed in order to producesatisfactorily shaped drainage channels extending a. substantialdistance from the borehole.

.act with the carbonate.

small volume of a relatively active acid, such as hydrochloric, toremove obstructions within the borehole and to enlarge the capillariesin the producing formation immediately adjacent the borehole. Followingthis step, the spent acid is flushed from the well and a larger volumeof acid solution of a very slow reaction rate is then injected into theformation. This slower reacting acid is forced into the formation asubstantial distance from the borehole and is allowed to re- The spentsolution obtained from the very slow reacting acid is then flushed fromthe formation. If the acid sludge cannot be satisfactorily removed bythe flushing step, it will be desirable to follow the second acidtreating step with a cleaning step in order to remove the acid sludgefrom the capillaries of the formation, so that fluid may flow readilytherethrough into the bore of the well. If desired, the sequentialtreatment described above may involve the use of strong acid in severalstages, and also the employment of weak acid in several stages.

It is to be understood that in the procedure as substantially constant.

The capillaries within the producing forma tions may also be enlarged tothe desired funnellike shape by reversing the sequence in which thecharges of different acids are used and initially employing a charge orcharges of a'very slowly reacting acid and following it with charges ofacids of increasing activity. When such aprocedure is employed, the slowacting acid is forced out into the formation a substantial distance fromthe borehole, and is allowed to react until it is spent. The very slowreaction of the acid causes it to enlarge the capillaries in the producing formation a substantially uniform amount as far as the acidpenetrates. The spent sludge is then removed from the formation and acharge of acid of greater activity forced into the borehole. This'acidwill not penetrate as far as the first acid, because its increasedactivity causes it to be spent before traveling this distance. Thecapillaries will accordingly be enlarged adjacent the borehole a greateramount than they are enlarged a considerable distance away from theborehole. The spent fluids are now removed from the well and. theprocedure repeated with acids of increasing activity.

The number of different stages in which the treatment is carried out maybe varied at the' option of the operator to' give optimum results.

.TJndersome circumstances a treatment with a slow reacting acid followedby a treatment of rapid reacting acid will be desirable, while underother conditions several separate treatments of the slow reacting acidfollowed by several treatments of the rapid reacting acid will beadvantageous. Often it will be desirable to use three or more acids ofdiffering strengths instead of only two acids, employing any of theprocedures outlined above with respect to the two acids. In using stagesof acid treatments it is advantageous, although not essential. toclean-the acid sludge from the formation after each stage to leave theformation clear for the subsequent acid step and for production afterthe acid treating is complete. For best results the capillary passagesin the formation should be generally funnel shaped and substantiallyfree from sludge when the treatment is completed.

A comparison of an embodiment of the present invention with aconventional acid treating process will now be given in connection witha discussion of the drawing of the present application.

In the drawing, the curve designated A represents a well treated with acommercial inhibited hydrochloric acid, and curve B represents anadjacent well in the same field treated in accordance with applicant'sprocess involving charges of acids of difierent activity.

Well A was treated as follows: The limestone producing formation was(packed off in the manner conventional to the art and 1,000 gallons ofinhibited hydrochloric acid forced into the producing formation. Theacidwas allowed to remain in position untl its reacton with the carbonateformation was complete, the acid sludge removed,. and a second chargeinjected. The second charge consisted of- 1,000 gallons of the sameacid, and after the reaction had been allowed to go to completion andthe acid sludge removed, .a third treatment of 2,000 gallons of the sameacid was given, followed by a cleaning step and a 4,000 gallon treatmentwith the acid. As will be seen from the curve, each charge increased theproductivity of the well, the total of 8,000 gallons of acid resultingin a productivity factor of 0.95.

Well B was given a preliminary treatment of 1,000 gallons of the sameinhibited hydrochloric acid used in. treating well A. After the sludgeresulting from this treatment was cleaned out of the well, two chargesof a mixture of monoand di-chlor-acetic acid mixture were used, a

first charge of 2,000 gallons, and a second charge of 4,000 gallons.After each injection of acid into well B, an attempt was made to removeacid sludge from the well, but the difficulty in removing the sludge wassuch that a substantial amount remained within the well and caused theproductivity of the well to be actually decreased.

laries of the formation. As shown by the curve, the cleaning step wassuccessful and increased the productivity of the well so that its finalproductivity factor was approximately 25% higher than that of well A.

- The drawing clearly shows that the utilization tivity factor isplotted against volume of formation dissolved in order to properlycompare the treatments of wells A and B. The points on the curveindicate the addition of the charges of acid to the wells. A givenvolume of the monoand di-chlor-acetic acid mixture used in treating wellB will, of course, not dissolve as much carbonate formation as the samevolume of commercial inhibited hydrochloric acid. The drawing indicatesthat when suificient acid is employed in each well to dissolve 90 cu.ft. of formation that approximately 20% greater increase in productivityfactor is obtained by utilizing the slow reacting acid, as abovedescribed.

The drawing also clearly shows that substantially no improvement couldbe expected in the treatment of well A by employing increased amounts ofhydrochloric acid, because the productivity curve has leveled off sothat it is substantially parallel with the axis.- It will be apparentthat, if desired, additional, treatments of slowly acting acids maybegiven well B and will result in an increased productivity factor becausethe end of curve I: is sloping sharply upward, having a shape similar tothe curve of well A after its treatment with only enough hydrochloricacid to dissolve 45 cu. ftof limestone formation.

' As has been previously suggested, it will sometimes be desirable toreverse the sequence of treatments and inject slow acting acid into thewell first and follow it by another stage or treatment in which a moreactive acid is employed.

For example, instead of employing the monoand di-chlor-acetic acidmixture in the second stage, it may be used for a first stage treatment,and'hydrochloric acid as the second stage.

It will be apparent that I do not intend to limit the present inventionto the specific acids'listed above. Other acids which reactrelatively-rapidly with the formation, as well as other acids whichreact relatively slowly with the formation, may

' In order to clean the acid sludge from well B, after using the monoanddi-chlor-aceticacid mixture, a charge of 3,000 gallons of inhibitedhydrochloric acid was forced into the formation to aid in the removal ofsludge from the capilbe employed in carrying out the .process.

It 'will also be evident that the present invention is by no meanslimited to the two stages above described. Under some conditions alarger number of stages 'will b desirable. A specific example of asequence of acids which have been found useful for such stage-operationis as follows, the acids being listedin order of increasing activity andionization constant:

Acid Ionization constant at 25 0.

Ka=1.38 10- at 25 C. Ka-1.56 10-' Ka-aoxlw Ka-2.0Xl0- Ks largHydrochloric acid While I have disclosed examples of specific materialswhich have been i'ound suitable for practicing the present invention,these examples are given by way of illustrationonly and are' aaaasaa Iclaim: i 1. A method oftreating a petroleum producing water-insolublecarbonate formation comprising the steps of preliminarily treating saidformations with commercial inhibited hydro-- chloric acid, removing the.products of reaction from the formation, injecting an acid capable ofdissolving the water-insoluble carbonates and having an ionizationconstant of a -lower order of magnitude than said hydrochloric acid insaid formation, removing the products of the reaction from saidformation; and producing said well.

2. A method in accordance with claim 1 in which the slowly reacting acidis a mixture of 'monoand di-chlor-acetic acids.

3. A process in accordance with claim 8 in which the first acid used isa mixture of monoand di-chlor-acetic, and the second acid used ishydrochloric acid.

4. A method of treatingunderground waterinsoluble carbonate formationsadjacent a bore bonates and having a relatively large ionizationconstant, removing the products of the reaction "from the formation,subsequently treating said formation with a second acid capable ofdissolving carbonates and having an ionization constant of a lowerorderof magnitude than first acid and removing the products of thereaction from the formation.

6. A method of treating a petroleum producing water-insoluble carbonateformation comprising the steps of treating the formation in stages andemploying in said stages acids capable of dissolving water-insolublecarbonates, the acids used in successive stages having ionizationconstants of progressively increasing orders of magnitude a lower orderof magnitude than said first :acid into said formation, whereby passagesextending a substantial distance from the bore hole and increasing insize adjacent said bore hole are produoed in said formation, removingsubstantially all of the products of the reaction from the formationafter .each acid treating step, and subsequently producing said well.

8. A method of treating a petroleum producing water-insoluble carbonateformation comprising the steps of injecting a charge of a first acidcapable of dissolving water-insoluble carbonates and removing theproducts of the reaction from the formation after each stage.

7. A method of treating a petroleum producing water-insoluble carbonateformation penetrated by a bore hole comprising the steps of separatelyinjecting a. first acid capable of dissolving waterinsoluble carbonatesand having a relatively large ionization constant into said formationand a carbonates and having anionization constant of i second acidcapable of dissolving water-insoluble and having an ionizationconstantof a low order of magnitude into said formation and forcing it asubstantial distance from the bore hole, allowing the acid to react withthe formation until it becomes spent, removing products of said reactionfrom the formation, subsequently injecting into said formation a chargeof a second acid capable of dissolving water insoluble carbonates andhaving an ionization constant of a higher order of magnitude than saidfirst acid, allowing it to react therewith, removing the products ofreaction from the formation, and producing said well.

9. A method of treating a petroleum producing water-insoluble carbonateformation adjacent a bore hole comprising the steps of separatelyinjecting charges of several acids into said formation, said acids beingcapable of dissolving waterinsoluble carbonates and having ionizationconstant of progressively increasing orders of magnitude, allowing eachcharge to become spent, removing the products of the reaction after eachacid charge-is injected and before the subsequent charge is added to thewell, removing the productsof the reaction after the last charge hasbeen spent, and producing petroleum from said formation.

10. A method of treating a petroleum producing water-insoluble carbonateformation adjacent a bore hole comprising the steps of separatelyinjecting charges of several acids into said formation, said acids beingcapable of dissolving water-insoluble carbonates and having whim-- tionconstants of progressively decreasing orders ofmagnitude, allowing eachcharge to become spent in the formation, removing the products ofreaction after each acid charge is injected and before the subsequentcharge is added to the well, removing the products of the reaction afterthe last charge has become spent, and producing petroleum from saidformation. i

ll. Amethod'in accordance with claim 1 in which the slowly reacting acidis a mixture of monoand dichloracetic acids and in which the products ofthe reaction of this acid with the formation are removed by treating theformation with commercial inhibited lwdrochloric acid.

I NEWTON B. DISMUES.

